The present invention relates generally to an organic EL (electroluminescent) device, and specifically to a device which emits light upon the application of an electric field to a thin film of multilayered structure. More specifically, the present invention is concerned with an organic electro-luminescent light emission device including a hole injecting layer comprising a specific triarylamine polymer, so that low driving voltage, stable light emission, high quality display images and high heat resistance are achievable.
An organic EL device comprises a thin film containing a fluorescent organic compound interleaved between an electron injecting electrode (cathode) and a hole injecting electrode (anode), and emits light making use of light emissions (fluorescence and phosphorescence) upon deactivation of excitons which are generated by injecting electrons and holes in the thin film for their recombination.
Features of the organic EL device are that surface light emission of high luminance of the order of hundreds candelas/m2 to scores of thousands of candelas/m2 is achievable at a low voltage of up to 10 V, and blue to red light emissions are achievable by selecting the type of fluorescent material.
On the other hand, problems with the organic EL device are that its light emission life is short, and its storage robustness and reliability are low for the following reasons.
(1) Physical Changes of Organic Compound
An inhomogeneous organic thin film interface caused as by the growth of crystal domains is responsible for a drop of the ability of the device to inject holes, short-circuiting, and dielectric breakdown. Especially when a low-molecular compound having a molecular weight of 500 or less is used, crystal grains manifest themselves and grow greatly, resulting in a strikingly reduced film property. When an organic thin film interface with ITO or the like is roughened, too, crystal grains manifest themselves and grow greatly, resulting in a light emission efficiency drop and current leakage which eventually lead to non-emission. This is also a leading cause of local non-emitting spots, i.e., dark spots.
(2) Oxidation and Delamination of Electron Injecting Electrode
To facilitate injection of electrons, for instance, Na, K, Li, Mg, Ca, and Al has so far used as a metal having a low work function. However, when these metals react with atmospheric moisture or oxygen or the organic layer peels off the electron injecting electrode, it is impossible to inject holes. Especially when a high-molecular compound is formed into film as by spin coating, solvent residues after film formation, moisture and decomposed products accelerate the oxidation reaction of the electrode, and causes the delamination of the electrode, resulting in local non-emitting spots.
(3) Low Light Emission Efficiency and Generation of Much Heat
The generation of heat is unavoidable because a current is passed through the organic compound and so the organic compound must be placed in a high field strength. The heat then gives rise to the melting, crystallization, and thermal decomposition of the organic compound, resulting in a deterioration and breakdown of the device.
(4) Photochemical, and Electrochemical Changes of Organic Compound Layer
Upon passing a current through the organic material, the organic material degrades, resulting in defects such as current or exciton traps. These defects in turn cause a deterioration of the device such as a driving voltage increase or a luminance drop.
Practical light emitting devices are used in various environments. Especially when such a device is used in high-temperature environments, the quality of display images drops or the device breaks down because of the crystallization or physical change of the organic compound or the rearrangement, i.e., migration, dispersion, etc. of the organic molecules.
A hole or electron injecting electrode interface that is an interface between an organic material and an inorganic material, especially the hole injecting electrode interface has a great influence on the film property of the organic material layer during film formation. In some cases, several problems arise; an inhomogeneous organic layer is formed on the hole injecting electrode, and no good interface can be formed.
For this reason, it has so far been reported to use on the hole injecting electrode interface in an organic EL device materials such as phthalocyanine, polyphenylenevinylene, evaporated polythiophene film, and amine polymer. However, it is found that the use of phthalocyanine (U.S. Pat. No. 4,720,432 or JP-A 63-295695) is not preferable because it yields a device which can be well operated in an initial state, but dark spots, light emission variations, etc. manifest themselves while it is operated over an extended period. This is because the phthalocyanine accelerates the crystallization of a material placed thereon due to its own microcrystalline nature. For polyphenylenevinylene, it is required to use a wet process such as spin coating wherein atmospheric impurities such as moisture are entrained therein, or ionic impurities such as leaving groups are entrained therein upon conversion from its precursor. Thus, the oxidation of the electrode proceeds rapidly, causing a striking luminance drop or a noticeable driving voltage increase.
Problems with the evaporated polythiophene film are that the reproducibility of fabrication of good devices is low due to large variations in the degree of polymerization of polythiophene and large fluctuations in polythiophene during evaporation, and the surface of ITO cannot be fully denatured due to difficulty involved in making polythiophene thick because the polythiophene itself has light absorption in a visible light region. Regarding amine polymers, for instance, dendrimer materials (JP-A 4-308688), tetraamine materials (U.S. Pat. No. 439,627) and triamine materials (JP-A 8-193191) have been reported. However, it is found that sufficient heat resistance is not obtained, and that a homogeneous and stable film cannot be obtained on a hole injecting electrode during high-temperature storage.
It is one object of the invention to provide an organic EL device which uses an optical and electronic non-structural material particularly less susceptible to physical changes, photochemical changes and electrochemical changes, and can give out light emissions of various colors with high reliability and high light emission efficiency.
Another object of the invention is to provide an organic EL device comprising an organic thin film formed by an evaporation technique of a compound that has high amorphism and high compatibility with a hole injecting electrode, said organic EL device being free from a driving voltage increase or a luminance drop and a current leakage with neither development nor growth of local non-emitting spots while it is driven, and so being capable of emitting light with high luminance, and high reliability such as high heat resistance.
Yet another object of the invention is to provide an organic EL device using a multilayered film, said organic EL device comprising a hole injecting electrode or an organic material combined therewith, to which the optimum work function is imparted, and having high heat resistance.
A further object of the invention is to provide an organic EL device capable of having high Hole mobility and so obtaining much higher current density.
Such objects are achievable by the inventions defined below as (1) to (10).
(1) An organic EL device comprising organic compound layers, at least one of which has a skeleton represented by formula (I):

 
where L0 is any one of o-, p-, and m-phenylene groups which have two, three or four rings and which may have a substituent with the proviso that when L0 is a phenylene group having four rings, the phenylene group may have an unsubstituted or substituted aminophenyl group somewhere therein, R01, R02, R03 and R04 are each any one of the following groups:

 
where R11, R12, R13, R14, R15, R16 and R17 are each a substituted or unsubstituted aryl group, and r1, r2, r3 and r4 are each an integer of 0 to 5 with the proviso that r1+r2+r3+r4xe2x89xa71.
(2) The organic EL device of (1), wherein a set of phenylene groups represented by L0 is a 4,4xe2x80x2-biphenylene group.
(3) An organic EL device comprising at least two organic compound layers, wherein the organic compound layer recited in (1) or (2) is an organic compound layer having a function of injecting and transporting holes.
(4) An organic EL device comprising three or more layers including at least an organic compound layer having a function of injecting holes and at least an organic compound layer having a function of transporting holes, wherein:
the organic compound layer recited in (1) or (2) is an organic compound layer having said function of injecting holes.
(5) The organic EL device of (3) or (4), wherein at least one layer of said organic compound layers includes a light emitting layer containing a hole transporting compound and an electron transporting compound.
(6) The organic EL device of (5), wherein said light emitting layer exists between the organic compound layer having a function of injecting holes and/or the organic compound layer having a function of transporting holes and the organic compound layer having a function of transporting electrons and/or an organic compound layer having a function of injecting electrons.
(7) An organic EL device comprising a hole injecting electrode, and including at least an organic compound layer having a function of injecting and transporting holes as recited in (3), an organic compound layer having a function of transporting holes, a light emitting layer, and an electron injecting electrode laminated on said hole injecting electrode in the described order.
(8) An organic EL device comprising a hole injecting electrode, and including at least an organic compound layer having a function of injecting holes as recited in (4), a light emitting layer, and an electron injecting electrode laminated on said hole injecting electrode in the described order.
(9) The organic EL device of any one of (3) to (8), wherein said organic compound layer having a function of injecting holes has a thickness of at least 100 nm.
(10) The organic EL device of any one of (5) to (9), wherein said layer containing said compound has a Hole mobility of at least 1.0xc3x9710xe2x88x923 cm2/Vs.
With the organic EL device of the invention wherein the compound represented by formula (I) is used for a hole injecting layer or a hole injecting and transporting layer, it is possible to achieve consistent and uniform light emission due to an improved thin film property. The organic EL device of the invention also remains stable or uncrystallized over a period of 1 year or longer in the air. The aforesaid compound is characterized by having in its molecular structure a phenylenediamine skeleton to optimize hole injection efficiency and a benzidine skeleton (biphenyldiamine) and a skeleton having diamines with respect to a plurality of phenylenes to improve Hole mobility. Further, the organic EL device of the invention can stand up to high-temperature driving, and can efficiently emit light at a low driving voltage and a small driving current. Furthermore, when the organic EL device of the invention is continuously driven, there is only a slight or limited increase in the driving voltage. It is here to be noted that the EL device of the invention has a light emission maximum wavelength of the order of 400 to 700 nm.